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A. Banerjee

Publications and source records attributed to A. Banerjee.

At least 55 records · Page 3Linked to original sources
arXiv2021

AstroSat Observation of Non-Resonant Type-C QPOs in MAXI J1535-571

Galactic transient black hole candidate (BHC) MAXI J1535-571 was discovered on 2017 September 02 simultaneously by {\it MAXI}/GSC and {\it Swift}/BAT instruments. It has also been observed by India's first multi-wavelength astronomy-mission satellite {\it AstroSat}, during the rising phase of its 2017-18 outburst. We make both the spectral and the temporal analysis of the source during 2017 September 12-17 using data of {\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) in the energy range of $3-40$~keV to infer the accretion flow properties of the source. Spectral analysis is done with the physical two-component advective flow (TCAF) solution-based {\it fits} file. From the nature of the variation of the TCAF model fitted physical flow parameters, we conclude and confirm that the source was in the intermediate spectral state during our analysis period. We observe sharp type-C quasi-periodic oscillations (QPOs) in the frequency range of $\sim 1.75-2.81$~Hz. For a better understanding of the nature and evolution of these type-C QPOs, a dynamic study of the power density spectra is done. We also investigate the origin of these QPOs from the shock oscillation model. We find that non-satisfaction of Rankine-Hugoniot conditions for non-dissipative shocks and not their resonance oscillations is the cause of the observed type-C QPOs.

astro-ph.HE↗
arXiv2021

String cosmology in Bianchi I space-time

Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of state.

gr-qc↗
arXiv2021

Bianchi II, VIII, and IX viscous fluid cosmology

In this paper we study the exact solutions for a viscous fluid distribution in Bianchi II, VIII, and IX models. The metric is simplified by assuming a relationship between the coefficients and the metric tensor. Solutions are obtained in two special cases: in one, an additional assumption is made where the matter density and the expansion scalar have a definite relation and in the other a barotropic equation of state between the matter density and the thermodynamic pressure is assumed. While the Bianchi II solutions are already found in the literature the other two classes of solutions are apparently new.

gr-qc↗
arXiv2021

Homogeneous Anisotropic Cosmological Models with Viscous Fluid and Magnetic Field

The paper presents some exact solutions of Bianchi types I, III and Kantowski-Sachs cosmological models consisting of a dissipative fluid along with an axial magnetic field. A barytropic equation of state between the thermodynamic pressure and the matter density, together with a pair of linear relations between the matter density, the shear scalar, and the expansion scalar have been assumed for simplicity. The solutions are basically of two different types, one for the Bianchi-I and the other for Bianchi-III and Kantowski-Sachs type. The presence of the magnetic field, however, does not change the fundamental nature of the initial singularity.

gr-qc↗
arXiv2021

Bianchi Type-II Cosmological Model with Viscous Fluid

A spatially homogeneous and locally rotationally symmetric Bianchi type-II cosmological model under the influence of both shear and bulk viscosity has been studied. Exact solutions are obtained with a barotropic equation of state between thermodynamics pressure and the energy density of the fluid, and considering the linear relationships amongst the energy density, the expansion scalar and the shear scalar. Special cases with vanishing bulk viscosity coefficients and with the perfect fluid in the absence of viscosity have also been studied. The formal appearance of the solutions is the same for both the viscous as well as the perfect fluids. The difference is only in choosing a constant parameter which appears in the solutions. In the cases of either a fluid with bulk viscosity alone or a perfect fluid, the barotropic equation of state is no longer an additional assumption to be imposed; rather it follows directly from the field equations.

gr-qc↗
arXiv2021

Bianchi type I cosmological model with a viscous fluid

Bianchi I cosmological models consisting of a fluid with both bulk and shear viscosity are studied. It is shown how the dynamical importance of the shear and the fluid density change in the course of evolution. Exact solutions with an equation of state $ρ= p$ for a stiff fluid are also obtained in several special cases, assuming the viscosity coefficients to be the power functions of the density. The results are, in some relevant cases, compared with those of Belinskii and Khalatnikov (1976) in the asymptotic limits and are seen to agree with them in that the models start with $ρ= 0$ at the beginning and evolve with the creation of matter by the gravitational field, finally approaching the Friedmann universe.

gr-qc↗
arXiv2021

Zeeman-driven parity transitions in an Andreev quantum dot

The Andreev spectrum of a quantum dot embedded in a hybrid semiconductor-superconductor interferometer can be modulated by electrostatic gating, magnetic flux through the interferometer, and Zeeman splitting from in-plane magnetic field. We demonstrate parity transitions in the embedded quantum dot system, and show that the Zeeman-driven transition is accompanied by a 0-π transition in the superconducting phase across the dot. We further demonstrate that flux through the interferometer modulates both dot parity and 0-π transitions.

cond-mat.mes-hall↗
arXiv2020

Ehlers as EM duality in the double copy

Given a solution to 4D Einstein gravity with an isometry direction, it is known that the equations of motion are identical to those of a 3D $σ$-model with target space geometry $SU(1,1)/U(1)$. Thus, any transformation by $SU(1, 1) \cong SL(2,\mathbb{R})$ is a symmetry for the action and allows one to generate new solutions in 4D. Here we clarify and extend recent work on electromagnetic (EM) duality in the context of the classical double copy. In particular, for pure gravity, we identify an explicit map between the Maxwell field of the single copy and the scalars in the target space, allowing us to identify the $U(1) \subset SL(2, \mathbb{R})$ symmetry dual to EM duality in the single copy. Moreover, we extend the analysis to Einstein-Maxwell theory, where we highlight the role of Ehlers-Harrison transformations and, for spherically symmetric charged black hole solutions, we interpret the equations of motion as a truncation of the putative single copy for Einstein-Yang-Mills theory.

hep-th↗
arXiv2020

Bilayers and quasi-3D stacks of Jain series fractional quantum Hall states from parton construction

We present a description of bilayers and quasi-three dimensional stacks of Jain series of fractional quantum Hall states using their parton descriptions, and argue for them as candidate states when the interlayer Coulombic interaction is comparable to the intralayer Coulomb interaction. For the bilayers, a K-matrix theory is presented and shown to be different from decoupled or bonded layers. The quasi-3D systems have gapless gauge excitations and gapped partons that may be able to move about in the 3D, thus presenting a toy model for quantum Hall-like states in three dimensions.

cond-mat.mes-hall↗
arXiv2020

Giant electron-phonon coupling of the breathing plane oxygen phonons in the dynamic stripe phase of La$_{1.67}$Sr$_{0.33}$NiO$_4$

Doped antiferromagnets host a vast array of physical properties and learning how to control them is one of the biggest challenges of condensed matter physics. La$_{1.7}$Sr$_{0.3}$NiO$_4$ (LSNO) is a classic example of such a material. At low temperatures holes introduced via substitution of La by Sr segregate into lines to form boundaries between magnetically ordered domains in the form of stripes. The stripes become dynamic at high temperatures, but LSNO remains insulating presumably because an interplay between magnetic correlations and electron-phonon coupling localizes charge carriers. Magnetic degrees of freedom have been extensively investigated in this system, but phonons are almost completely unexplored. We searched for electron-phonon anomalies in LSNO by inelastic neutron scattering. Giant renormalization of plane Ni-O bond-stretching modes that modulate the volume around Ni appears on entering the dynamic charge stripe phase. Other phonons are a lot less sensitive to stripe melting. Dramatic overdamping of the breathing modes indicates that dynamic stripe phase may host small polarons. We argue that this feature sets electron-phonon coupling in nickelates apart from that in cuprates where breathing phonons are not overdamped and point out remarkable similarities with the colossal magnetoresistance (CMR) manganites.

cond-mat.str-el↗
arXiv2020

Non-equilibrium magnetic response of canonical spin glass and magnetic glass

Time and history dependent magnetization has been observed in a wide variety of materials, which are collectively termed as the glassy magnetic systems. However, such systems showing similar non-equilibrium magnetic response can be microscopically very different and they can be distinguished by carefully looking into the details of the observed metastable magnetic behavior. Canonical spin glass is the most well studied member of this class and has been extensively investigated both experimentally and theoretically over the last five decades. In canonical spin glasses, the low temperature magnetic state obtained by cooling across the spin glass transition temperature in presence of an applied magnetic field is known as the field cooled (FC) state. This FC state in canonical spin glass is widely believed as an equilibrium state arising out of a thermodynamic second order phase transition. Here, we show that the FC state in canonical spin glass is not really an equilibrium state of the system. We report careful dc magnetization and ac susceptibility measurements on two canonical spin glass systems, AuMn (1.8%) and AgMn (1.1%). The dc magnetization in the FC state shows clear temperature dependence. In addition, the magnetization shows a distinct thermal hysteresis in the temperature regime below the spin glass transition temperature. On the other hand, the temperature dependence of ac susceptibility has clear frequency dispersion below spin glass transition in the FC state prepared by cooling the sample in the presence of a dc-bias field. We further distinguish the metastable response of the FC state of canonical spin glass from the metastable response the FC state in an entirely different class of glassy magnetic system namely magnetic glass, where the non-equilibrium behavior is associated with the kinetic-arrest of a first order magnetic phase transition.

cond-mat.str-el↗
arXiv2020

Nature of Magnetic Excitations in the High-Field Phase of $α$-RuCl$_3$

We present comprehensive electron spin resonance (ESR) studies of in-plane oriented single crystals of $α$-RuCl$_3$, a quasi-two-dimensional material with honeycomb structure, focusing on its high-field spin dynamics. The measurements were performed in magnetic fields up to 16 T, applied along the [110] and [100] directions. Several ESR modes were detected. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identify most of the observed excitations. Most importantly, we show that the low-temperature ESR response beyond the boundary of the magnetically ordered region is dominated by single- and two-particle processes with magnons as elementary excitations. The peculiarities of the excitation spectrum in the vicinity of the critical field are discussed.

cond-mat.str-el↗
arXiv2020

The field cooled state of canonical spin-glass revisited

Canonical spin-glass (SG) is an enigmatic system in condensed matter physics. In spite of the intense activities of last five decades several questions regarding the nature of the SG phase transition and the SG ground state are yet to be resolved completely. In this backdrop we have revisited the field cooled state of canonical spin-glass. We have experimentally studied magnetic response in two canonical spin-glass systems AuMn(1.8%) and AgMn(1.1%), both in the field cooled (FC) as well as zero field cooled (ZFC) state. We show that the well known magnetic memory effect, which clearly established earlier the metastable nature of the ZFC state in SG, is also present in the FC state. The results of our experimental study indicate that the FC state also is a non-equilibrium state, and hence the energy landscape involved is a non-trivial one. This in turn seriously questions the picture of spin-glass transformation as a second order thermodynamic phase transition.

cond-mat.str-el↗
arXiv2020

Light-driven permanent transition from insulator to conductor in Ga2O3

The transition from insulator to conductor can be achieved in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a mechanism the lattice may adopt to induce such a transition. We show that limited exposure to sub-bandgap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide Ga2O3 with 9-orders of magnitude increase in electronic conduction. Photoexcitation modifies the charge state of an O-vacancy and the redistribution of the localized electrons, leading to a massive structural distortion in the lattice that is shown to be the underlying mechanism. It modifies density of states and introduces new stable states with shallower energy levels, leading to this intriguing behavior. We suggest that this mechanism may occur in other wide bandgap metal oxides leading to drastic modification in their electronic properties.

cond-mat.mtrl-sci↗
arXiv2020

Magnetic and transport properties in pyrochlore iridates (Y$_{1-x}$Pr$_x$)$_2$Ir$_2$O$_7$: The role of $f$-$d$ exchange interaction and $d$-$p$ orbital hybridization

The $f$-$d$ magnetic exchange interaction is considered to be a key ingredient for many exotic topological phases in pyrochlore iridates. Here, we have investigated the evolution of structural, magnetic and electronic properties in doped pyrochlore iridate, (Y$_{1-x}$Pr$_x$)$_2$Ir$_2$O$_7$. Apart from geometrical frustration, pyrochlore iridates are well known for its active spin-orbit coupling effect. The substitution of Pr$^{3+}$ (4$f^2$) for the nonmagnetic Y$^{3+}$ (4$d^0$) acts as a magnetic doping, which provides an ideal platform to study $f$-$d$ exchange interaction without altering the Ir-sublattice. With Pr substitution, system retains its original cubic structural symmetry but the local structural parameters show an evolution with the doping concentration $x$. The robust magnetic-insulating state in Y$_2$Ir$_2$O$_7$ is drastically weakened, while Pr$_2$Ir$_2$O$_7$ ($x$ = 1.0) shows a paramagnetic-metallic behavior. A metal-insulator transition is observed for $x$ = 0.8 sample. This evolution of magnetic and electronic properties are believed to be induced by an exchange interaction between localized Pr-4$f$ and itinerant Ir-5$d$ electrons as well as by an increased hybridization between Ir-$t_{2g}$ and (basal) O-$p$ orbitals as observed in XAS study. The resistivity in insulating materials follows a power-law behavior with a decreasing exponent with $x$. A negative magnetoresistance is observed for present series of samples at low temperature and where the magnetoresistance shows a quadratic field dependence at higher fields.

cond-mat.str-el↗
arXiv2019

Irrotational Bianchi V Viscous Fluid Cosmology with Heat Flux

The irrotational Bianchi V cosmological model under the influence of both shear and bulk viscosity, together with heat flux, has been studied. Exact solutions for the model are obtained with three physically viable assumptions. The first two relate the matter density, shear scalar, and expansion scalar and the third is a barotropic equation of state, connecting the matter density and thermodynamic pressure. The properties of the solutions are studied and the temperature distribution is also presented explicitly. It has been observed that along with the viscosity, heat flux further adds to the rate of entropy increase.

gr-qc↗
arXiv2019

Scaling behavior of Charge order melting in Magnetic field-Pressure-Temperature space of 2.5% Al doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$

Dc magnetic measurements across the charge ordering (CO) transition temperature (T$_{CO}$) in polycrystalline Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ have been performed under simultaneous influence of external hydrostatic pressure (P) and magnetic field (H). We show the first experimental evidence that the melting of charge order instability obey an interesting scaling function, $δ$T$_{CO}$/P$^α$ = $f$(H/P$^β$) in H-P-T landscape, where $δ$T$_{CO}$ is the suppression of T$_{CO}$ by P and H. Corresponding values of the exponents, $α$ = 1.63 and $β$ = 0.33 have been extracted from data collapsing phenomena. Possible origin of such a scaling behavior has been discussed.

cond-mat.str-el↗